In these arms, the first cartridge is usually loaded in the firing chamber by pulling back the obturator carriage manually against the resistance of a return spring or other elasticated means of the loader. Then, when released, the obturator carriage moves forward to push the cartridge into the chamber and close the barrel-obturator carriage. Each successive cartridge enters the chamber similarly, due to the action of the compressed gases that are caused by the cartridge which has been fired. This action causes the backward movement of the obturator carriage, a barrel recoil which is shorter than that of the obturator carriage and the extraction of the used cartridge case.
On the front part of the obturator carriage there is usually a sight to help the operator get a better aim of the target. The barrel is a separate element and is guided by a certain clearance on the obturator carriage; therefore, a poor alignment between the barrel and the obturator carriage when they are in the firing position will cause a reduced precision in the arm, since the barrel is free to move up or down, or to right or left.
To rectify this, a variety of precision devices for firearms have been proposed, one of which described in U.S. Pat. No. 869,581. This well-known device consists of a bush, screwed on to the front end of the barrel and having a conical external surface, which is designed to fit a conical hole made in the front part of the obturator carriage.
However, in this action, the centring and the alignment stop between barrel and obturator carriage are entrusted exclusively to the external conical surface of the bush and the internal surface of the hole in the obturator carriage, given that there is no positive stop or block for the barrel against the upper surfaces of the oburator carriage. Furthermore, the conical bush, continually stressed by use of the gun, is subject to surface wear and has the tendency to come unscrewed. These are conditions which over time cause modifications in the action and the precision of the bush, with a consequent loss of accuracy.
The same applicant has also proposed a perfected precision device, which is the subject of a patent request in Italy, corresponding to the publication of European Patent N.O. 697 578.
This precision device is characterized by the fact that on the front part of the barrel, externally, there is a threaded end part and following this, towards the back of the barrel, there is a cylindrical section on which is mounted a conical alignment bush. The latter is designed to fit axially with the aforementioned conical hole in the obturator carriage, and on this threaded section are screwed the means for axial blocking of the bush, which include a regulating ring that will rest against both the alignment bush and the obturator carriage and a threaded nut for blocking the said regulating ring.
However, in this design, a certain unreliability has been noted in blockage of the alignment bush, because of the tendency of the ring to come unscrewed, and, furthermore, in order to compensate for wear of the bush it is necessary to move the latter axially, by adjusting the regulating ring and the blocking nut, as necessary.